The invention relates to forming inorganic, metal oxide spheres from molded microparticles.
A variety of methods are currently used to produce glass beads, which are also referred to as glass microspheres. These processes often require repeated steps of pulverizing and classifying particulate material in an effort to obtain glass beads that exhibit a relatively narrow size distribution. Some glass bead manufacturing processes include generating a particulate feed material, followed by conversion of the particulate feed material to a glass by melting. The particulate feed materials can be formed by pulverizing glass particles (or components that form glass when heated at a sufficiently high temperature) and intimately blending, e.g., by milling, the pulverized particles with a volatile liquid such as water. A binder such as dextrin or starch is sometimes added to bind together the milled raw material particles. The slurry of milled material is then dried, e.g., in bulk or by spraying the composition into a dry atmosphere maintained at an elevated temperature to yield dried feed material. In the case of spray-drying, the dried agglomerates can then be converted directly into glass. Feed material dried in bulk often takes the form of a cake. A dried cake can be converted to useful agglomerates by pulverizing. Optionally, the cake can be sintered before pulverizing to pre-react some components of the cake. In the case of bulk drying, the pulverized agglomerates must be classified to achieve a sufficiently narrow size range of finished beads. Classification of the agglomerates is undesirable, due to added cost and energy usage.
Once the classified agglomerates have been generated, glass microspheres can then be formed using a variety of melting methods. In one melting method, the agglomerates of raw material are passed through a flame having a temperature sufficient to melt the particles and through a distance sufficient to spheroidize the melted particles. For many raw materials exposure to a flame having a temperature of from about 1500° C. to about 2000° C. is sufficient. The melted particles are then quenched, e.g., in air or water, to form solid beads. The quenched particles optionally can be crushed to form particles of a smaller desired size for the final beads and then further processed. In other methods, the raw material is melted and the melted material is poured continuously into a jet of high velocity air. Molten droplets form as the jet impinges on the liquid stream. The velocity of the air and the viscosity of the melt are adjusted to control the size of the droplets. The molten droplets are then rapidly quenched, e.g., in air or water, to form solid beads. Beads formed by such melting methods are normally composed of a vitreous material that is essentially completely amorphous (i.e., noncrystalline). The beads are often referred to as “vitreous,” “amorphous,” or simply “glass” beads or microspheres. Beads formed by liquid glass atomization often exhibit a wide size distribution, requiring classification (e.g., screening) of the product, which leads to excess cost and energy use.
These processes often require many steps of pulverizing, classifying, or sintering to achieve particles having a desired size and size distribution.